The STEN was a family of British 9mm sub machine guns used extensively by British and Commonwealth in WW2 and the Korean War.
They were simple in design and cheap to produce. The Sten used simple stamped metal components and minor welding, which required a minimum of machining and manufacturing. Their simplicity of production meant that their components could be fabricated in small, dispersed workshops and then then borough to gather for final assembly at the Enfield site. It was a popular weapon with allied commando units and counter insurgency and resistance groups.
STEN is an acronym, from the names of the weapon’s chief designers, Major Reginald V. Shepherd and Harold Turpin, and EN for Enfield.
Over 4 million Stens in various versions were made in the 1940s.
Mark II was the most common variant, with two million units produced.